Magnetic recording media are widely used as a recording tape, video tape, computer tape, floppy disc or the like. A magnetic recording medium of such a type fundamentally has a structure having a magnetic layer, which is obtained by dispersing a fine ferromagnetic powder in a binder, as laminated on a nonmagnetic support.
In general, a magnetic recording medium needs to exhibit high performance capability in a wide range of different types of characteristics, such as electromagnetic characteristic, running durability and running efficiency. In particular, with recent popularization of 8 mm-video, high-band 8 mm-video and S-VHS video tape recorders, a strong requirement has emerged for video tapes for use in such recorders which have especially excellent electromagnetic characteristics. For example, video recording tapes having a high video output and having an excellent original-reproducing ability are demanded.
Several conventional methods have been used to improve the electromagnetic characteristics of magnetic recording media. Above all, a method for improving the characteristics of the fine ferromagnetic powder, which is a magnetic recording substance in the media, is straight forward and effective. Accordingly, the fine ferromagnetic powder to be used in the recording media has been pulverized into an increasingly finer powder for the purpose of attaining high density recording with such fine powder. Suitable types of fine ferromagnetic powder materials include a modified iron oxide, such as iron oxide modified with one or more hetero atoms such as cobalt, which has come to be used in place of the non-modified iron oxide. Additionally, ferromagnetic metals such as iron, nickel or cobalt as well as alloys of such metals have come to be used.
By use of such improved fine ferromagnetic powders, it was thought that production of magnetic recording media having good electromagnetic characteristics would be more easily attainable. In fact, however, it is difficult to produce magnetic recording media having improved electromagnetic characteristics despite the improvements in fine ferromagnetic powders. This failure can be attributed to the following reasons. Specifically, the dispersibility of a fine ferromagnetic powder in a binder is lowered with any corresponding reduction of the grain size of the powder. Additionally, with respect to the characteristics of fine ferromagnetic powders, the dispersibility thereof in a binder becomes even lower in the following order: for example, .gamma.-iron oxide, cobalt-deposited .gamma.-iron oxide and fine powders of ferromagnetic metals. Therefore, the attempted improvements in fine ferromagnetic powders to be incorporated into the magnetic layer of recording media would often yield a result opposite to that desired. For example, the dispersion condition of the fine ferromagnetic powders in the magnetic layer would be adversely affected. As a consequence, the excellent characteristics of the improved fine ferromagnetic powders could not be displayed sufficiently.
Various conventional approaches have been proposed and actually carried out for improving the dispersibility of fine ferromagnetic powders in a binder to be used in the magnetic layer of recording media. For instance, improvement of binders has been illustrated in JP-B-58-41565, and JP-A-57-44227, JP-A-59-30235, JP-A-60-238306, JP-A-60-238309, JP-A-60-239371 and JP-A-61-172213 (the terms "JP-A" and "JP-B" as used herein refer to a "published unexamined Japanese patent application" and an "examined Japanese patent publication", respectively), where binders containing a polar group(s) such as --SO.sub.3 M, --OSO.sub.3 M, --PO.sub.3 M.sub.2, -OPO.sub.3 M.sub.2, --COOM, --NR.sub.2 or --N.sup.+ R.sub.3 X.sup.- or binders containing both the polar group and epoxy ring(s) are used. The illustrated binders have a strong adsorbing power to fine ferromagnetic powders, and, therefore, the powders can be more effectively dispersed into the binders. However, even in the case of such improved binders, dispersion of fine ferromagnetic powders in these binders is found to be difficult if the grain size of the powders has been reduced for the purpose of attaining better high density recording. As a result, in that instance, the excellent electromagnetic characteristics of the fine ferromagnetic powders could not be displayed sufficiently. Therefore, development of a technique has been desired which enables the electromagnetic characteristics of fine ferromagnetic powders to be sufficiently displayed in the above mentioned high dispersing binders.
There is a known method of conducting kneading dispersion of the components used in producing a magnetic layer-coating composition for an extended period of time for the purpose of improving the dispersion condition of fine ferromagnetic powders in the coating composition. However, since a relatively large shear stress is imparted to the fine ferromagnetic powder during kneading dispersion thereof, the method has the drawback of causing deterioration of the intrinsic characteristics of the fine ferromagnetic powder itself. Additionally, the method demands lengthy periods of time for producing a magnetic recording medium, and, therefore, has another drawback insofar manufacture efficiency and manufacturing cost considerations.
In order to overcome these problems, other approaches have been proposed, which include a method of using a surface-treated fine ferromagnetic powder as treated with a surface-treating agent such as a silane coupling agent; a method of using a dispersing agent such as fatty acids or phosphates; and a method of using a binder as prepared by adding a low molecular epoxy resin to a system comprising a vinyl copolymer and an urethane prepolymer (as described in JP-B-56-23210).
Where a fine ferromagnetic powder is surface-treated by the above mentioned method of using a silane coupling agent or using a dispersing agent, the affinity of the surface of the thus treated fine ferromagnetic powder with a binder is improved and the dispersing rate of the powder in the binder is accelerated. However, as the fine ferromagnetic powder and the binder are not strongly bonded to each other in this instance, the dispersibility of the powder, including the dispersion stability thereof, in the binder is still insufficient.
Where a fine ferromagnetic powder is dispersed in the presence of a low molecular epoxy resin or epoxy group-containing compound along with a hardening agent of a polyisocyanate compound for the purpose of improving the mechanical strength of the magnetic layer to be formed and for improving the running durability of the same layer, the epoxy group would tend to be cleaved during the kneading and dispersing step in preparing the intended magnetic coating composition such that the resulting --OH group would react with the polyisocyanate compound present to thereby extremely shorten the pot life of the coating composition. This drawback is experienced during the process of preparing the coating composition.
On the other hand, with respect to the grain size of a fine ferromagnetic powder, if the size is extremely small, the powder have a large specific surface area. As a result, the thickness of the binder needed to cover the fine ferromagnetic powder becomes thin if the amount of the binder added to the powder is not correspondingly increased. In this instance, the breaking elongation of the magnetic layer containing the powder is relatively small, and, therefore, the magnetic layer is rendered extremely brittle. Where the magnetic layer is too brittle, the magnetic layer existing on the edge parts of the tape often would be cracked with ease. Further, the physical interaction between the finished magnetic layer and any parts of a recorder/playback machine would cause the magnetic layer to be broken and peeled off to cause a problem of an increase of dropouts. Additionally, there would be another problem in that the magnetic layer would be peeled off at the tape side edge portions as a result of the slitting step performed in preparing the magnetic tapes to also cause increase of dropouts.
In order to overcome these problems, a method of reducing the amount of the polyisocyanate compound to be added, or, alternatively, of further adding a plasticizer, may be employed. However, the method is effective only for preventing the brittleness of the magnetic layer at the expense of other needed characteristics. For example, the method causes an increase in the temperature dependence of the mechanical strength property of the magnetic layer and especially causes reduction of the mechanical strength of the layer at a high temperature. As a result, the running durability of the tape is thereby lowered.
Under these circumstances, development of a technique of highly dispersing a fine ferromagnetic powder having an extremely small grain size and a technique of producing a magnetic recording medium for high density recording, which may realize an extremely excellent electromagnetic characteristic and an extremely excellent running durability with little dropouts, has been highly desired.